Photo electric tube



July 10, 1934,

Filed Nov. 28. 1951 INVENTORS' JAN HENDRIK DE BOER AND MARTEN CORN 1s T EVE5 ATTORNEY Patented July 10,

PHOTO ELECTRIC TUBE Jan Hendrik dc Boel' and Marten Cornelia rem, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application November In Germany 28, 1931, Serial No. 577.740 December 6, 1930 5. Claims. (0]. 250-275) g the photo-electric metal and for this purpose In the main patent specification No. 452,785 it has been proposed to provide the photo-electric substance of the light sensitive electrode of a photo-electric tube on a layer containing a chemical compound and electrically-conducting particles. As set forth in the said specification, these particlesgmay be particles of the photo-electric substance and it is advisableto provide in the layer, in addition to the particles of said photoelectric substance, particles oi another conducting material. By a photo-electric substance is meant hereinafter a substance which emits electrons when exposed to radiation.

The invention has for its object to increase the sensitivity of such a photo-electric tube i. e.

the current produced in the tube due to a deterelectric metal. The fllm of photo-electric metal present on the said layer is preferably made very thin; it has for example, a thickness 0! one or a few atoms. The layer supporting this photo electric metal i. e. the above-described mixed layer, should preierably be thick, for example thicker than 100 molecules. I

As the photo-electric metal use is preferably made of the alkali metals although other substances, ior example the alkaline earth metals such as barium may be utilized as well.

In the manuiacture oi such a photo-electric tube one pr'eierably proceeds as follows. A layer oi! metal oxide, for example silver oxide, is first produced within the tube, which may be effected by oxidizing a metal layer produced, for example, by evaporation in a vacuum. The metal oxide layer may also be obtained, however, in another manner, ior example by evaporating a metal oxide present on an incandescent body arranged within the tube. After the iormation of the metal oxide layer and, if desired, after the exhaustion of the tube, an excess of photo-electric metal, for example oi alkali-metal, is introduced into the tube. which while not connected to a vacuum pump is heated to such a temperature that the metal oxide is reduced and the oxide of the photo-electric metal. for example alkali oxide, is formed. The excess of photoelectric metal is given the opportunity of penetrating into the produced mixture oi particles of the reduced metal and particles of the oxide 0! it is advantageous slightly to heat the tube, for example to about 100 C. During this heating a film of alkali metal deposits on the layer produced.

In reducing the metal oxide care should be so taken to prevent the tube from being heated to such an extent that the oxide of the photo-electric metal decomposes or volatilizes or that the metal particles produced clot together.

In order that the invention may be clearly as understood and readily carried into eiiect it will be explained more fully with reference to the accompanying drawing which represents, by way 0!. example, a photo-tube according to the inven- 7 3 tion. In the drawing:'

Figure 1 is a section of this photo-electric tube and I Figure 2 represents a detail thereof.

The photo-electric; tube shown in Figure 1 has a spherical wall 1 to which is sealed a tube 2 provided with a stem 3 which carries the electrode 4 of the photo-electric tube. As distinctly appears from Figure 2 this electrode consists of a nearly closed metal ringthe ends of which are connected to current supply wires 5 and 6. The wall 1 is coated with a silver layer 7 which is electrically connected to a current supply wire 8. The silver layer 7 is covered with a layer 9 consisting of intimately mixed silver particles, caesium particles and cesium oxide particles, this intermediate layer 9 being provided with a caesium film 10.

In manufacturing the tube, first a silver layer is formed on thewall. For this purpose, some silver may be secured to the electrode 4 prior to the arrangement of the latter in the tube, said quantity of silver being evaporated by heating the electrode 4 after the tube has been exhausted, which may be effected with the aid of a vacuum pump connected to the exhaust tube 11. The evaporated silver settles on the wall of the tube, a portion 12, of said wall being kept free irom silver deposit by means of a screen not shown in the drawing. This portion 12 constitutes the window through which during operation the rays of light penetrate into. the tube. The silver layer may also be formed in other ways than by evaporation in a vacuum, for example by precipitation from a silver compound.

After the silver layer has been formed, oxygen, for example in the form 01' air, is admitted into the tube and an electric discharge is established between the electrode 4 and the silver layer, ow-

oxidized. lhe oxidation should preferably be continued until a rather thick layer of silver oxide of uniform structure has been formed.

Subsequently, the tube is exhausted again and shut off from the pump whereupon caesium is introduced in excess into the tube, for example by distillation from a receptacle (not shown in the drawing) which is connected to a tube 13. The caesium may also be developedwithin the tube for example by heating a mixture of a caesium compound and a suitable reducing agent. When after the introduction of the caesium into the tube the latter is heated to about 200 C., the silver oxide is reduced and caesium oxide is formed. Care should be taken, that during this heating operation the tube is not connected to the vacuum pump as otherwise the caesium would be withdrawn from the tube. Furthermore, the temperature must not be raised too much. If, for example, the tube would be heated to 250 C., the casium oxide, which has the composition 0520, would volatilize and at a higher temperature it would even be decomposed.- Besides, at that high temperature the silver particles would melt together so that they would no longer be present in the layer in a finely divided state.

During and after the formation of the mixture of silver particles and caesium oxide the excess of caesium is given the opportunity of penetrating into the mixture formed. This penetration can be facilitated by heating the tube, for example to 100 C., the tube being during this heating operation not in communication with a vacuum pump. A layer is thus obtained which contains silver particles, ceeesium particles and caesium oxide particles in a mixed state.

Part of the cmsium settles as a thin film on this intermediate layer. If within the tube there should still be some free caesium this may be removed from the tube, if desired, by connecting I the latter to a vacuum pump and by exhausting cold. Alternatively, it is possible to connect the tube to a side-tube which is subsequently cooled and sealed off, or to introduce into the tube a substance adapted to absorb the excessive caesium, for example finely divided carbon. Usemay also be made of a chemical compound capable of taking up the excessive caesium, for example of lead-oxide. For this purpose for example the stem of the tube may be made of lead glass.

It is evident that the photo-electric substance may consist not only of caesium but also of other alkali metals or other substances, for example barium, while the intermediate layer may also contain particles of a metal other than silver. The tube is preferably used as a high vacuum tube but it is also possible to provide it with a gaseous filling.

The photo-electric tube according to the invention has a very great sensitivity. In highvacuum tubes it is'possible for example, to obtain currents of 40 micro-amperes per lumen. These are currents being about ten times as strong as those reached in the high-vacuum tubes according to the main patent. Besides, the tubes can be made so as to be perfectly reproducible so that they are suitable for mass-production.

What we claim is:

1. In the manufacture of a photoelectric tube comprising an evacuated envelope enclosing an anode and a cathode having a surface which contains silver, the process which consists in oxidizing the silver containing surface of said cathode, evacuating said envelope, producing in said evacuated envelope an atmosphere of alkali metal vapor, heating said cathode in said atmosphere to reduce some of the silver oxide on the surface of said cathode to metal without preventing the formation of oxide of the alkali metal on said surface, holding said cathode at a temperature slightly below the vaporizing point in said envelope of the alkali metal for a time sufficient to permit said alkali metal to deposit from said atmosphere on the surface of said cathode and penetrate said surface, and removing the excess of alkali metal vapor. V

2. The method of producing a light sensitive coating on a silver surfaced cathode of a photoelectric tube having an envelope enclosing said cathode, which consists in oxidizing the surface of said cathode, evacuating and temporarily sealing the envelope of the tube, producing in the sealed evacuated envelope an atmosphere containing alkali metal vapor, heating said cathode in said atmosphere to a temperature sufiicient to reduce to particles of silver some of the silver oxide on the surface of said cathode and insuflicient to vaporize oxide of said alkali metal from said surface, lowering the temperature of said cathode to slightly below the temperature at which the alkali metal deposits on said cathode from said alkali metal vapor atmosphere, holding said cathode at said lower temperature for a time to permit some penetration of said surface by said alkali metal, and removing the ex.- cess alkali metal vapor.

3. In the manufacture of a photoelectric tube comprising an evacuated envelope enclosing a cathode havingg surface which is predominantly silver, the method of rendering said cathode light sensitive which consists in oxidizing said cathode surface, producing in said evacuated envelope an atmosphere containing casium vapor, heating said cathode in the presence of said atmosphere to about 200 C. and below 250 C. until some of the oxidized silver is reduced to finely divided metal, maintaining said cathode at about 100 C. to produce a thin film of casium on said surface, and removing the excess of caesium.

4. A photoelectric tube comprising an envelope enclosing an anode and a cathode comprising a.

metal base having on its surface a mixture of particles of the metal of the base, of an alkali metal, and of oxide of said alkali metal in the form of a thin layer, and'a film of said photoenclosing an anode and a cathode comprising a silver plate having its surface coated with a relatively thick layer of a mixture of finely divided silver, of caesium oxide, and of particles of caesium, and a relatively thin film of caesium on said layer.

JAN HENDRIK DE BOER. MARTEN CORNELIS TEVES. 

